Compression, timing and octane primer - Mustang Evolution

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Old 11-21-2013, 12:13 AM   #1
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Compression, timing and octane primer

When I bought my new '14 V6 the other day I was surprised to hear the salesman tell me it ran on regular old 87 octane gas. It surprised me because of the relatively high compression ratio of these motors at 10.5:1. Every other motor I've owned with CR like that required 93 octane gas(all imports).

So I've done some searching and there is a lot of misinformation out there on the Mustang boards... Some people think there is no advantage to running premium, some do. Some people think compression is variable LOL.

I'm here to tell you that a 10.5:1 compression engine will make more power with higher octane. Why? Because the computer can give the engine more spark advance which will burn more fuel and make more power. 87 octane is perfectly acceptable and will get you the factory rated HP/TQ numbers because the ECU will limit the amount of timing due to the fuel. The old Honda's I was racing didn't have such advanced ECU's yet and running 87 octane piss water would cause them to ping and detonate especially under hard throttle.

I found a video that AM had done where they dynoed a car on 87 and then on 93 and if I remember right the car made 6 or 7 HP more! You may laugh at 6 HP but how much did that $300 CAI give you? I'm a hotrodder at heart and 6 HP is 6HP, I'll take it thank you very much!

Also, the premium fuels usually contain more/better detergents than regular. For example, Shell V-Power. LOVE THAT STUFF. Amoco Ultimate is also very nice, although all Amoco gas now has Invigorate in it.

Here is an excerpt from an old article in the June 2010 issue of Muscle Mustangs and Fast Fords written by Jim McCraw:

"Ford also worked outside the engine to make the package a better fit for the Mustang. The powertrain ECU has been upgraded with a very aggressive deceleration cylinder shutoff for fuel economy, coupled with very rapid tip-in for street performance. On the flip side, the ECU has been reprogrammed with adaptive-knock spark control. If the two knock sensors embedded in the cylinder block don’t hear knocking, the ECU will keep advancing the spark until it does."

Check the last sentence there. "The ECU will keep advancing the spark until it does." Well if the ECU is going to give you more spark until it senses knocking... DIng ding ding! Higher octane fuel will get you more advance because it is less prone to detonation because it burns slower.

That's the thing about octane the average car owner does not know/understand. A higher octane fuel burns slower than a lower one. That's why dummys who put it in their 9:1 CR civics get ****ty mileage. A whole bunch of unburned fuel is going out their tail pipes.

Here is a link to the whole article, it's a pretty good read on the tech in our 3.7l engines:
Ford Mustang 3.7L V6 Engine Explained | A 3.7L V6 Mustang Owner Resource for Modifications and Repairs
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Old 11-21-2013, 01:25 AM   #2
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Good read sir. Very informational and appreciated. Ive always ran 91 Shell v-power because its better fuel for the engine and now because my car is tune to utilize the higher octane to the max performance wise
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Old 11-21-2013, 06:02 AM   #3
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From the 2011 owners manual, page 290:

"Your vehicle will run normally on 87 octane regular fuel without
damaging the engine, but premium fuel with an octane rating of 91
(R+M)/2 or higher is recommended for best overall performance."


Many folks on this forum have used 91 and 93 octane, and experienced no real "seat of the pants" performance upgrade, and the cost of 91 or 93 octane did not warrant the small increase that may have been given by upgrading to these fuels. I ran Shell and Mobile 87 octane for 43K miles, and my Mustang ran great. After installing a tune, I bumped to the 93 Octane required, and it runs better, but I account that to the tune combination.

Now with a tune, it's a whole different story.
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